Synthetic rutile process b

ABSTRACT

A process for recovering titanium as synthetic rutile from a titaniferous ore, for example a secondary ilmenite, includes the steps of treating the ore in a reducing atmosphere at elevated temperature above 1075° C. in the presence of a carbonaceous reductant whereby to convert the ilmenite to reduced ilmenite in which iron oxides in the ilmenite have been reduced to metallic iron, and separating out the metallic iron so as to obtain a synthetic rutile product. The carbonaceous reductant comprises a coal selected for a moisture content below 40%, a volatiles content greater than 30%, ash content below 10%, and a gasification reactivity that results in an increased rate of reduction of iron oxides and titanium species effective to achieve a TiO2 content of 90% or greater in the synthetic rutile product.

FIELD OF THE INVENTION

This invention relates to the recovery of titanium as synthetic rutilefrom titaniferous ores, and is particularly though not exclusivelydirected to improving the economics of recovery of titanium from lowergrade secondary or altered ilmenites.

BACKGROUND OF THE INVENTION

The standard process by which titanium dioxide is recovered from theilmenite component of Western Australian mineral sands deposits is theBecher reduction process in which the ilmenite is roasted in a rotarykiln in the presence of coal and a reducing atmosphere so as to reduceiron oxides in the ilmenite to metallic iron, which is then separated byaqueous oxidation to obtain a product known as synthetic rutiletypically having a TiO₂ content of 90% or greater. The synthetic rutileis a feedstock for further processing to white paint pigment and otherapplications. These further processes are sensitive to a minimum TiO₂content, and the output of the Becher process is in turn dependent on arelatively tight ilmenite feed specification, e.g. in Western Australiaan iron content measured as FeO<12%. In practical terms this limits thefeedstock for the Becher process to secondary ilmenites, also known asaltered or weathered ilmenites.

The restrictive ilmenite specification for the Becher process isbecoming a more urgent problem in locations where secondary ilmeniteresources are diminishing in respect of their TiO₂ grade. For example,the standard feed specification for Western Australia secondary ilmeniteto the Becher process is FeO<12%, 57%<TiO₂<65%. From the perspective ofthe owners of these resources, it has been and remains desirable toextract greater or more economically attractive commercial returns forthe resource, and/or to extend the life of lower grade secondaryilmenite provinces.

Whilst the ilmenite properties cannot be changed, the coal propertiescan still have a significant overall effect. The volatile component ofcoal is necessary in the ilmenite preheating stage (<800° C.) whereminimal reduction occurs. The char produced from the preheating stageloses its volatile component and the internal structure becomes porousin the process. The internal porosity structure created is measured interms of micro and macro porosity. A finely porous structure has asignificantly increased surface area to volume ratio which has beenshown to increase reaction rates. The internal structure created duringde-volatilisation largely depends on the volatile content and themolecular coal structure.

The function of the coal is however two-fold so improvements inperformance in one aspect may result in a reduction in the other. Whilsta highly porous char structure assists reaction rates in the reductionzone the loss of carbon and moisture during thepreheating/de-volatilisation stage leads directly to a reduction incarbon mass in the reduction zone. A trade-off therefore occurs betweenthe amount of volatiles produced in the preheat zone, the charporosity/reactivity and the amount of carbon entering the reductionzone.

Western Australian Collie coal has long been considered as having theideal properties as a fuel and a reductant for ilmenite reduction kilns.Its qualities include a relatively low ash content of 6%, a low volatilecontent of 26% and high ash fusion temperature of 1410 deg(deformation).

Collie coal is from the Gondwanan coal formation (the super continentforming part of Antarctica, Australia, South Africa and India some 240to 280 million years ago). Coals of this origin are unique in that theyare of relatively low rank for their age due to being buried atrelatively shallow depths.

In the coalification process vegetative and organic matter (peat) isfirst compressed to remove moisture to less than 70% to form brown coalor lignite. Coals with high moisture (eg Victorian brown coal) aretherefore usually low rank with poor mechanical strength. In the secondcoalification stage volatiles are converted to fixed carbon and theremaining moisture is further reduced to less than 50% whilst the colourdarkens appreciably. Such coals (eg Collie coal) are classified assub-bituminous having a specific energy value greater than 19 MJ/kg andvolatile content less than 30%. There are another two stages whichinvolve further reduction of moisture and volatiles. Bituminous coalshave a further reduction in moisture to less than 4% (eg Sydney andBowen Basins). Anthracite is formed in the very last stage of volatileremoval and usually occurs only in tectonic zones. The only Australiancoals approaching anthracite composition are Yarabee and Baralaba inQueensland.

Bituminous coals are considered less suitable for the Becher process dueto their low volatile content and higher ash levels (>8%).

Victorian Brown coal briquettes trialled in 1997 and 1998 showed onlyminor process improvements. No significant benefits were measured duringthese trials.

It is an object of this invention, at least in one or more embodiments,to provide one or more modifications to the standard Becher process thatimprove the economics of recovery of titanium into synthetic rutile.

It is another object of the invention, at least in one or moreembodiments, to improve the economics of recovery of titanium from lowergrade secondary or altered ilmenites.

Reference to any prior art in the specification is not, and should notbe taken as, an acknowledgment or any form of suggestion that this priorart forms part of the common general knowledge in Australia or any otherjurisdiction or that this prior art could reasonably be expected to beascertained, understood and regarded as relevant by a person skilled inthe art.

SUMMARY OF THE INVENTION

In accordance with the invention, it has been surprisingly found thatthe objects of the invention can be met, at least in part, by theemployment of a sub-bituminous or lignite coal reductant having agasification reactivity that results in an increased rate of reductionof iron oxides and titanium species.

The invention provides a process for recovering titanium as syntheticrutile from a titaniferous ore, for example a secondary ilmenite,including the steps of:

-   -   treating the ore in a reducing atmosphere at elevated        temperature above 1075° C. in the presence of a carbonaceous        reductant whereby to convert the ilmenite to reduced ilmenite in        which iron oxides in the ilmenite have been reduced to metallic        iron, and separating out the metallic iron so as to obtain a        synthetic rutile product,    -   characterised in that said carbonaceous reductant comprises a        coal selected for a moisture content below 40%, a volatiles        content greater than 30%, ash content below 10%, and a        gasification reactivity that results in an increased rate of        reduction of iron oxides and titanium species effective to        achieve a TiO₂ content of 90% or greater, preferably at least        93%, in said synthetic rutile product.

It may be that the gasification reactivity of the coal is sufficientlyhigh to achieve said TiO₂ content, but a high value for the gasificationreactivity may not be sufficient. It may be relatively high as a coalgasification reactivity, by which is meant in the context of thisspecification significantly higher than the average of all coals. Inpractical terms, this means that the gasification reactivity is towardsthe higher end of the range of gasification reactivity generally foundin coals. The gasification reactivity is preferably greater than 0.005g-g/min at 850° C., more preferably greater than 0.01 g-g/min at 850°C., both values for coal char at atmospheric pressure. Alternatively oradditionally the gasification reactivity is preferably at least twicethat of typical Collie coal, more preferably at least three times thatof typical Collie coal.

The elevated temperature of said treatment is preferably in the range1075-1200° C., more preferably between 1100 and 1075° C., and mostpreferably in the range 1100 to 1150° C.

One known indicator of higher coal gasification reactivity is the levelof ion-exchanged calcium, although it is thought that other impurityelements can play a similar role. The selected coal accordinglypreferably has impurity levels of ion-exchanged inorganic elementssufficiently high to increase the gasification rate of the coal thusimproving the reducing conditions in the process and thereby increasingthe rate of reduction of iron oxides and titanium species. Such elementsmay include alkaline earth elements such as calcium and magnesium, oralkali elements such as sodium, or iron. Coal containing relatively highlevels of ion-exchanged calcium has been found to be particularlyuseful.

A measure of sufficiently high levels of ion-exchanged inorganicelements is the acid extractable proportion of the elements: this isdesirably greater than 50%, more preferably greater than 70%, mostpreferably greater than 80%. Usefully, at least one such inorganicelement is present to the extent of at least 0.2% db on a dry coalbasis.

While the coal may of any rank including bituminous, a suitable coal istypically a sub-bituminous or lignite coal.

Moisture content of the coal may be a total moisture content between 5and 40%, or an inherent moisture content in the range 5 to 25%, ineither case preferably not less than 5%. In the latter case, themoisture content is preferably 20% or less. Volatiles content ispreferably >40%. Ash content is preferably <5%. Ultimate hydrogencontent, on a dry ash basis, is preferably greater than 4%. Ultimatecarbon content is preferably greater than 65%. Ash fusion temperaturemay be above 1100° C., on an initial deformation temperature (I.D.T.)basis, above 1200° C. on a hemispherical temperature (H.T.) basis (morepreferably at least 1150° C. and 1250° C. respectively).

Preferably, char is mixed with the ilmenite before it is delivered forthe aforesaid treatment step. The presence of char mixed with theilmenite has been found to further assist in reducing the rate ofagglomeration or sintering arising from reoxidation.

Preferably, the sulphur content of the coal is less than 1% w/w, morepreferably less than 0.5%, most preferably less than 0.2%. Preferably,there is no additional sulphur present for most of the duration of saidtreatment. It has been found that sulphur contained in the coal abovethese preferred levels (for example by providing a blend of low-sulphurand high sulphur coal fractions) or present by virtue of additionalsulphur, adversely affects the reactivity of the ilmenite, i.e. the rateof metallisation (the speed at which iron oxide is converted to metalliciron in the reduction treatment step).

Thus, if in order to further increase the TiO₂ content of the syntheticrutile product of the process, it is desired to deliver sulphur to theilmenite during said treatment step, e.g. for removing manganeseimpurity as manganese sulphide, such delivery is effected only laterduring the duration of the reduction treatment, for example only duringthe last 3 hours of a 9 hour treatment.

The iron content of the ilmenite, expressed as FeO, is preferably in therange FeO<12%.

Preferably, free oxygen in the treatment atmosphere is no greater than2.5% and preferably less than 2%, most preferably less than 1%.

Preferably, the treatment at elevated temperature in a reducingatmosphere is carried out in an inclined rotary kiln of the kindnormally employed for the Becher process. The material recovered fromthe lower end of the kiln is known as reduced ilmenite, a mix ofmetallic iron and titanium dioxide with a residual content of iron andother impurities. This reduced ilmenite is cooled to prevent reoxidationof metallic iron and then passed to the separation step.

The separation step may be any suitable separation method employed inBecher reduction processes. A typical such method is an aqueousoxidation step in which the metallic iron is oxidised or rusted tomagnetite, haematite or lepidocrocite in a dilute aqueous solution ofammonium chloride catalyst. An alternative or additional separation stepmay entail an acid leach or wash, typically employing sulphuric acid.

EXAMPLES

To establish the relative performance of different coals a standard potreduction test was used. For this test 500 g samples of a secondarystandard Capel ilmenite (FeO 12%) were combined with 500 g of coal andreduced for 9 hours under a standard heating profile which reaches andmaintains 1100° C. by 6.5 hours. Coal was prepared by crushing andscreening to +4 mm and −9.5 mm.

Samples of reduced ilmenite (RI—the product of the treatment prior toseparation of the metallic iron) were extracted at timed intervals of3.5, 4.0, 4.5, 5.0, 5.6, 6.2, 6.8, 7.4 and 9.0 hrs. The extractedsamples were then analysed for metallic iron and total iron. The final9.0 hr RI sample in each case was acid leached in 1.0M sulphuric acid atroom temperature for 15 minutes and then at 60° C. for a further 60minutes.

A total of 10 coals were selected for testing including Collie coal asthe reference. Collie coal is commonly used in Western Australia as thesolid reductant in commercial operations of the standard Becher processusing secondary or altered ilmenites. Coal specifications for each ofthe coals selected are shown in Table 1, while Table 2 sets out an assayfor the standard Capel secondary ilmenite.

Metallic iron levels were measured at the intervals discussed above, andobserved metallisation rates for the different coals are shown as logcures in FIG. 1.

Using the log reducibility constant derived from FIG. 1 (slope of logmetallisation curve), kiln feed rates were predicted via calculation ina 90 step kiln model and these are set out in FIG. 2. The baseline,throughput rate with Collie coal is predicted as 40.1 t/hr. Other testedcoals ranged from 37.7 to 49.3 t/hr, save for CS3 which ranked first byan easy margin at 68.3 t/hr.

RI samples from the pot reductions for each coal sample were acidleached at a strength of 1.0M sulphuric acid to simulate the SR gradeproduced.

XRF assays of the SR product are shown in Table 3. SR grades produced bythe test coals were on average 1.0% TiO₂ better than Collie coal. CS3coal produced an SR grade of 93.9% TiO₂ compared to Collie coal at 92.4%TiO₂.

In general better TiO₂ grades were generated by the slower of the testcoals (CS6, CS7 and CS9) where the amount of time for reoxidation at theend of the reaction is minimised. In reality the faster test coals suchas CS3 and CS2 would produce better SR TiO₂ grades if the RI wasextracted at 6.2 hrs instead of 9.0 hrs.

Manganese (MnO) was notably higher in test coal reductions due to theirlower sulphur content.

The metallisation profile for CS4 showed that metallisation stopped midway through the reaction and did not complete. Iron extraction and TiO₂grades were observed to suffer accordingly.

Test work was now conducted on CS3 coal to determine what mightcharacterise its clearly better performance in the pot reductions. Thegasification (CO₂) reactivity behaviour of char samples (200-300 μm)produced from the CS3 coal and the Collie coal was determined using ahigh-pressure thermogravimetric analyser. For samples of about 300 mg,CO₂ reactivity was determined from the rate of sample mass loss due tothe reaction C+CO₂ (g)

2CO(g). Tests were performed under two temperature conditions atatmospheric pressure: isothermal at 850° C. and a varying temperatureincreased from 700° C. at a rate of 2° C./min. The latter test allowedthe temperature dependence of the gasification reaction to bedetermined.

The relative reactivities of the coal chars are presented in Table 4. Itwill be seen that CS3 coal was found to have a gasification reactivityat 850° C. about five times higher than Collie coal.

Elemental analyses of the coals (CS3 and Collie) are set out in Table 5.It will be seen that the reactive coal has materially higher levels ofcalcium and magnesium (a full order of magnitude difference) relative tothe Collie coal and this was found to be the case also in analyses ofthe respective ash residues. On a dry coal basis, each is above 0.2% db.It was established that the calcium and magnesium, and also the iron,were present in an ion-exchanged form in the reactive coal. This wasestablished by demonstrating that the acid extractable levels of Ca, Mgand Fe in the reactive coal were of the order of 85-95%, while theCollie coal had much lower levels of acid extractable Ca, Mg and Fe(less than 50%). The presence of ion-exchanged calcium, iron, sodiumand, to a lesser extent magnesium, in coals has been found to enhancethe gasification reactivity. By increasing the gasification rate of thecoal, the reducing conditions in the process are improved, therebyincreasing the rate of reduction of iron oxides.

One potential benefit of a more reactive coal would be to reduce thefeed coal ratio and so reduce the expense of coal. Reducing the feedcoal ratio helps to offset the additional coal cost but also negativelyimpacts on kiln capacity. A cost-benefit analysis is therefore needed tofind the optimum outcome.

Simulations were performed to estimate kiln throughput rates atdifferent feed coal ratios. Conditions could not be found where a feedcoal ratio of less than 0.35 could be achieved safely (with sufficientchar rates to prevent reoxidation). Attempts at further reductions inkiln temperature to reduce overall throughput rates to around 40 t/hrwere unsuccessful in achieving the required char production rates.Higher char rates were produced at higher throughput rates compared tolower throughput rates (due to the greater overall coal input), with a0.35 coal ratio giving the minimum acceptable result. Throughput ratesand feed coal ratio data are plotted in FIG. 3.

At the nominal 0.42 coal ratio the predicted feed rate was 68.3 t/hr. Ata 0.40 coal ratio (5% saving) the predicted feed rate was 67.8 t/hr. Ata 0.35 coal ratio (16% saving) the predicted rate was 66.2 t/hr. At a0.30 coal ratio (28% saving) the predicted rate was 64.1 t/hr but thisdoes not produce minimum acceptable char rates. At low char productionrates (coal ratios of 0.30 and lower), RI quality may become adverselyaffected through reoxidation.

It will be appreciated that identification of a basis for selecting aneffective higher reactivity coal for the synthetic rutile process givesrise to a variety of opportunities for economic improvements in theprocess. Lower TiO₂ grade secondary ilmenites may still be economicallyprocessed to synthetic rutile of acceptable grade. Coal feed ratios maybe reduced, as discussed above, where good quality ilmenite feed remainsavailable. Alternatively, coal ratios may be maintained to achievehigher throughput rates for given or higher TiO₂ SR grade.

FIG. 4 is provided for illustrative purposes to demonstrate howgasification reactivity can affect reduction rates. The figureillustrates the rates of reduction of iron oxides (as measured bymetallic iron formation) and titanium species, for respective kilnreductions of an ilmenite under similar conditions with Collie coal andthe reactive coal. An assay of the ilmenite employed is provided underthe graph. Although the ilmenite here was not a secondary ilmenite (FeOis above 12%), the TiO₂ content is high because of low other speciessuch as Si and Al, and the depicted comparative behaviour of the coalsis valid across a wide range of ilmenites.

It will be understood that the invention disclosed and defined in thisspecification extends to all alternative combinations of two or more ofthe individual features mentioned or evident from the text or drawings.All of these different combinations constitute various alternativeaspects of the invention.

TABLE 1 Coal Specifications (Typical specifications- as received fromCoal suppliers) Inherent Moisture Volatile Fixed Ash Fusion MoistureContent Ash matter Carbon Ultimate (%, daf) Temperature (deg C.) % [%ar] [% ar] [% ar] [% ar] Carbon Hydrogen I.D.T. H.T. Desired <10 <25<5 >40 >41 80 >5 >1200 >1250 Max <15 <10 >35 >75 Collie 27 27 6 26 41.074.9 4.6 CS1 10 18 4.5 38 47.5 76.7 5.3 1150 1210 CS2 14.5 25 1 42.542.0 74.3 5.6 1200 1260 CS3 16.5 24.5 2.5 43.5 37.5 77.4 5.8 1200 1250CS4 5 9.5 4 39 52.0 80.5 5.4 1200 1300 CS5 4.4 11 12 40.5 43.1 78.2 6.11450 1510 CS6 14 26 5 39.5 41.5 75.6 8.2 1200 1400 CS7 12 20 5 40 41.084.0 4.9 1200 1230 CS8 12 1.1 43.8 43.0 68.5 4.8 1260 1410

TABLE 2 ilmenite assay Standard Capel % FeO 14.0 TiO₂ 57.5 Fe₂O₃ 38.9SiO₂ 0.90 ZrO₂ 0.11 P₂O₅ 0.05 Al₂O₃ 0.62 Nb₂O₅ 0.16 Cr₂O₃ 0.04 MgO 0.22CaO <0.001 V₂O₅ 0.18 MnO 1.30 S 0.02 Th (ppm) 103 U (ppm) 11

TABLE 3 SR grades from test and Collie coal reductions on standard CapelSR ilmenite Reference SR Collie ilmenite Collie Coal SR Assay MC376 CoalRepeat CS1 CS2 CS2R CS3 CS4 CS5 CS6 CS7 CS8 CS9 TiO2 57.5 92.4 92.4 93.894.0 93.0 93.9 88.7 93.9 93.8 94.1 94.7 93.4 Fe2O3 38.60 6.01 5.42 2.812.90 3.75 3.43 7.40 2.90 3.09 2.88 2.50 3.48 SiO2 0.90 0.65 0.65 1.130.72 0.74 0.81 1.07 0.82 0.94 0.96 0.92 0.92 ZrO2 0.11 0.06 0.06 0.110.07 0.07 0.07 0.08 0.07 0.08 0.07 0.07 0.07 P2O5 0.05 0.02 0.02 0.020.03 0.03 0.04 0.01 0.03 0.01 0.01 0.01 0.01 Al2O3 0.62 0.77 0.77 0.880.83 0.84 0.88 0.91 0.88 0.88 0.93 0.87 0.91 Nb2O5 0.16 0.25 0.25 0.250.25 0.25 0.25 0.24 0.25 0.25 0.25 0.25 0.25 Cr2O3 0.04 0.08 0.08 0.130.13 0.12 0.10 0.10 0.11 0.14 0.11 0.14 0.13 MgO 0.22 0.36 0.36 0.410.40 0.41 0.43 0.40 0.41 0.41 0.42 0.44 0.42 V2O5 0.18 0.27 0.27 0.280.27 0.29 0.26 0.25 0.27 0.27 0.28 0.26 0.26 MnO 1.21 1.40 1.75 1.931.94 1.92 1.95 1.88 1.96 1.36 1.38 1.65 1.65 S 0.02 0.03 0.01 0.01 0.010.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 Th 103 123 113 157 152 121 100106 107 126 118 98 94 U 11 13 17 14 12 13 14 15 16 11 11 14 14 FeO = 14%

TABLE 4 Char-CO₂ Gasification Reactivity Char CO₂ Reactivity g-Activation Sample Description g/min @ 850° C. Energy kJ/mol Char fromReactive Coal 0.0113 226.7 Char from Collie Coal 0.00205 187.0

TABLE 5 Elemental Analysis (% dry coal basis) Collie Coal CS3 Coal % db% db Carbon 71.4 68.5 Hydrogen 4.1 4.9 Nitrogen 1.3 0.84 S_(total) 0.540.11 Cl_(total) 0.01 0.00 Si 1.06 0.34 Al 0.88 0.17 Fe 0.34 0.38 Ti0.086 0.014 K 0.031 0.02 Mg 0.02 0.22 Na 0.02 0.01 Ca 0.05 0.56

1. A process for recovering titanium as synthetic rutile from atitaniferous ore, for example a secondary ilmenite, including the stepsof: treating the ore in a reducing atmosphere at elevated temperatureabove 1075° C. in the presence of a carbonaceous reductant whereby toconvert the ilmenite to reduced ilmenite in which iron oxides in theilmenite have been reduced to metallic iron, and separating out themetallic iron so as to obtain a synthetic rutile product, wherein saidcarbonaceous reductant comprises a coal selected for a moisture contentbelow 40%, a volatiles content greater than 30%, ash content below 10%,and a gasification reactivity sufficiently high to result in anincreased rate of reduction of iron oxides and titanium specieseffective to achieve a TiO₂ content of 90% or greater in said syntheticrutile product.
 2. A process according to claim 1 wherein the elevatedtemperature of said treatment is in the range 1075-1200° C. 3.(canceled)
 4. A process according to claim 1 wherein said gasificationreactivity of the coal is relatively high (as defined herein).
 5. Aprocess according to claim 1 wherein the selected coal has relativelyhigh impurity levels of one or more ion-exchanged inorganic elementsthat increase the gasification rate of the coal thus improving thereducing conditions in the process and thereby increasing said rate ofreduction of iron oxides and ilmenite species.
 6. A process according toclaim 5 wherein the acid extractable portion of said one or moreion-exchanged inorganic elements is at least 50%.
 7. A process accordingto claim 1 wherein the selected coal has relatively high impurity levelsof ion-exchanged calcium.
 8. A process according to claim 1 wherein theselected coal is a sub-bituminous or lignite coal.
 9. A processaccording to claim 8 wherein inherent moisture content of the selectedcoal is 20% or less, volatiles content is >40%, and ash content is <5%.10. A process according to claim 1 further including mixing char withthe ilmenite before it is delivered for said treatment step.
 11. Aprocess according to claim 1 wherein the sulphur content of the coal isless than 1% w/w, and there is no added sulphur present for most of theduration of said treatment.
 12. A process according to claim 11, whereinthe sulphur content of the coal is less than 0.5%.
 13. A processaccording to claim 11, wherein the sulphur content of the coal is lessthan 0.2%.
 14. A process according to claim 11 further includingdelivering sulphur to the ilmenite during said treatment step forremoving manganese impurity as manganese sulphide, such delivery beingeffected only later during the duration of the reduction treatment. 15.A process according to claim 1 wherein the iron content of the ilmenite,expressed as FeO, is less than 12%.
 16. A process according to claim 1wherein free oxygen in the treatment atmosphere is no greater than 2.5%.17. A process according to claim 1 wherein the TiO₂ content achieved insaid synthetic rutile product is at least 93%.
 18. A process accordingto claim 5 wherein the selected coal is a sub-bituminous or lignitecoal.
 19. A process according to claim 18 wherein inherent moisturecontent of the selected coal is 20% or less, volatiles content is >40%,and ash content is <5%.
 20. A process according to claim 18 wherein theacid extractable portion of said one or more ion-exchanged inorganicelements is at least 50%.
 21. A process according to claim 11 whereinthe TiO₂ content achieved in said synthetic rutile product is at least93%.
 22. A process according to claim 5 wherein the iron content of theilmenite, expressed as FeO, is less than 12%.
 23. A process according toclaim 15 wherein the TiO₂ content achieved in said synthetic rutileproduct is at least 93%.